1. Field of the Invention
This invention is concerned with an optical disk memory system including a disk medium which has multiple information tracks alternating with single guide tracks and is concerned with apparatus for tracking and reading/writing information from the disk.
2. Description of the Prior Art
There are a number of known systems for reading and tracking information on optical disks or other optical media. In relatively more sophisticated systems, disks or other media of the contemplated type typically have information tracks which alternate with guide tracks as viewed along a radial line. The "tracks", information and guide, may each, in fact, be a single track in the optical disk which spirals around the disk from a location near to the disk axis location to a location remote therefrom much as the groove in a conventional audio record. In some relatively simple optical media systems, the information track and guide track are the same.
All such systems involve means for servoing or tracking along the guide track so that information is correctly read from the information track. The guide tracks in the optical medium are typically raised above or depressed into the medium surface by an amount which is typically .lambda./8n to .lambda./4n where .lambda. is the wavelength of an impinging light beam and n is the index of refraction of the media of propagation. Typical tracking systems involve the use of one tracking beam or two tracking beams which may be produced from one beam source. The illuminating beam is typically focused by a lens to a spot size whereby when the beam is centered over a track, 50% of the beam's power is in the track. Because of the approximate .lambda./4n wavelength step between the track and remainder of the medium surface, the power of the reflected beam when centered over a track is substantially reduced at the center of the pattern relative to the power of the beam applied to the medium and the power collected by the illuminating lens is similarly reduced.
One or two detectors are appropriately positioned to receive the reflected power and are applied to a servoing means to move the tracking beam to be centered on the guide track. A reading means is coupled to the means producing the illumination beam to thus be positioned over the information track which lies beside and a given distance from the guide track. In a conventional two-spot tracking system (which really involves a third spot for reading information), two illumination beams are provided though there may, in fact, only be one illumination source. In a two-spot tracker the two beams or spots are typically offset by an amount such that their 50% power levels just touch an imaginary line parallel to the tracks. As each of the beams passes over a guide track, due to the cancellation of signal caused by the approximately 1/4n wavelength depression or rise of the guide track, there is a much reduced signal. Electrical circuitry is then provided to subtract the return signal from one beam from the return signal from the other beam to produce a single sine wave or S curve having the center of the S curve mark the position when the two beams are an equal distance on either side of the center line of a guide track.